how to calculate knee point voltage of current transformer?

Answer Posted / rajkumar

Knee Point Voltage Vk = IFsc * K * (Rct+(2*RL)+Rr)
Where,
IFsc - Fault current at the secondary of CT
IFsc->IFprimary/CT Ratio
IFprimary -> Fault MVA at primary/(1.732*Primary Voltage)
K - Dimensioning Factor
Rct - CT Resistance
RL - Lead Resistance
Rr - Resistance of relay to which CT is connected

Knee point voltage is the voltage at which 10% raise in secondary voltage will result in 50% raise in current..

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